Research development is energetically promoted for a planer light wave circuit (PLC) configured with a silica glass waveguide which is formed on a silicon substrate. An arrayed waveguide grating (AWG) utilizing this PLC technology is a circuit realizing optical wavelength multiplexing/de-multiplexing and plays an important role as an optical communication component.
The AWG has a temperature dependence of a transmission wavelength for light to be multiplexed/de-multiplexed. This is because the effective refractive index of the silica glass waveguide configuring the AWG has a temperature dependence. Therefore, a typical AWG is required to have a temperature adjustment apparatus added for keeping constant wavelength transmission properties.
For omitting the temperature adjustment apparatus required to be added to the AWG, a method has been developed for reducing the temperature dependence of the transmission wavelength in the AWG. Details of this method are disclosed in Patent documents 1 and 2. The AWG having the reduced temperature dependence of the transmission wavelength is referred to as a temperature independent AWG or athermal AWG. The athermal AWG disclosed in Patent documents 1 and 2 is realized by means of forming a groove which is disposed so as to intersect with the propagation axis of a lightwave in each waveguide (arrayed waveguide or slab waveguide) within the AWG and by inserting material having a refractive index temperature coefficient different from that of the effective refractive index of the waveguide (hereinafter described as “temperature compensation material”) in the groove.
FIG. 40 is a plan view showing an exemplary configuration 4000 of an athermal AWG type optical wavelength multiplexing/de-multiplexing circuit of the conventional art which has a groove formed in a slab waveguide. The athermal AWG type optical wavelength multiplexing/de-multiplexing circuit 4000 is provided with a first input/output waveguide 4001, a first slab waveguide 4002, an arrayed waveguide 4003, a second slab waveguide 4004, a second input/output waveguide 4005, and a groove 4006 which is formed in the first slab waveguide 4002 and which is filled with the temperature compensation material.
Further, FIG. 41 is a diagram showing a cross-sectional structure of a part along the line XLI-XLI in the athermal AWG type optical wavelength multiplexing/de-multiplexing circuit 4000 shown in FIG. 40. The cross-sectional structure of a part along the line XLI-XLI includes a groove 4006, a silicon substrate 4007, a waveguide core 4008, and a clad 4009. The groove 4006 is formed by the removal of a part of the waveguide core 4008 and a part of the clad 4009 to divide waveguide core 4008.
The athermal AWG 4000 has a function of causing wavelength-multiplexed signal light input into the first input/output waveguide 4001 to be de-multiplexed into the second input/output waveguide 4005 for an output as signal light of each wavelength channel, and a function of causing signal light of each wavelength channel input into each waveguide of the second input/output waveguide 4005 to be multiplexed into the first input/output circuit 4001 for an output as wavelength-multiplexed signal light, resulting in operating as the optical wavelength multiplexing/de-multiplexing circuit.
Further, the groove 4006 is divided into plural grooves in FIG. 40, and this is because the plural grooves can reduce a radiation loss more than a single groove. In FIG. 40, the length Li of the i-th arrayed waveguide is expressed as Li=L1+(i−1)·ΔL and is designed so as to become sequentially longer by a certain amount ΔL. In correspondence with this design, in the first slab waveguide 4002, the sum of the lengths Li′ in which the light wave to be input into each of the arrayed waveguides is divided by the grooves 4006 is expressed as Li′=L1′+(i−1)·ΔL′ and formed so as to become sequentially longer by an amount ΔL′ proportional to ΔL. At this time, a center transmission wavelength λc from a center waveguide of the first input/output waveguide 4001 to a center waveguide of the second input/output waveguide 4005 in the AWG is expressed as follows.λc={naΔL′−nsΔL′+n′ΔL′}/M  Formula 1
Here, na is an effective refractive index of the arrayed waveguide, ns is an effective refractive index of the slab waveguide, n′ is an refractive index of the temperature compensation material, M is a diffraction order of the AWG, and naΔL−nsΔL′+n′ΔL′ indicates a distance difference between neighboring optical paths, that is, an optical path length difference in the AWG. At this time, it is assumed that n′ is close to ns and the refraction angle of light wave in the groove is sufficiently small. Here, the optical path length is a distance felt by the light wave and is obtained as a product of the refractive index of the material and a physical path distance. Also here, when α is an effective refractive index temperature coefficient of the arrayed waveguide and the slab waveguide (α=dna/dT=dns/dT, T: temperature) and α′ is a refractive index temperature coefficient of the temperature compensation material (α′=dn′/dT), the athermal AWG is configured as ΔL′/(ΔL−ΔL′)=−α/α′, that is, ΔL′=ΔL/(1−α′/α). Therefore, the temperature-induced change of the optical path length difference in the arrayed waveguide and the slab waveguide is cancelled by the temperature-induced change of the optical path length difference of the temperature compensation material filled in the groove, and the temperature dependence of the center transmission wavelength is compensated. The temperature compensation material may be any material having α′ which satisfies the above condition for α of the waveguide, but it is particularly preferable to use a material in which α′ and α have signs opposite to each other and also |α′| is sufficiently larger than |α|. This is because ΔL can be set to be smaller and excessive loss by the groove can be suppressed. The material satisfying such conditions includes, for example, a silicone resin which is an optical resin having α′ of approximately −35×α. Further, it is preferable to use the optical resin in that the optical resin has an excellent long-term reliability as an optical component material.
As another method for reducing the temperature dependence of the transmission wavelength in AWG, there is a method of cutting an AWG chip in an arc along a circuit, bonding a metal rod for connecting both ends of the chip, and causing the AWG chip to be deformed by the thermal expansion or contraction of the metal rod to cancel the temperature-induced change of the optical path length difference between the neighboring arrayed waveguides. Details of this method are disclosed by Non-patent document 1.
In addition, as still another method for reducing the temperature dependence of the transmission wavelength in the AWG, there is a method of dividing the input side or output side slab waveguide in an AWG chip, bonding the divided chips and a metal plate, and changing the relative position between the divided slab waveguides by the thermal expansion or contraction of the metal plate, to cancel the temperature-induced change of the optical path length difference in the arrayed waveguide.
Meanwhile, with the progress of the optical communication system, a system such as a ring network or a mesh network has started to be built for connecting multi points and switching a communication line flexibly. In such a high level network, light signal is required to pass through the multi points without being demodulated into an electric signal, and the optical wavelength multiplexing/de-multiplexing circuit to be used is required to have a high flatness of a transmission spectrum and a low loss. As the optical wavelength multiplexing/de-multiplexing circuit having such excellent transmission properties, a MZI-synchronized AWG type optical wavelength multiplexing/de-multiplexing circuit combining the Mach-Zehnder interferometer circuit (MZI) and the AWG is proposed. Details of this circuit are disclosed by Patent documents 3 and 4. By this MZI-synchronized AWG having a low loss and also a flat transmission spectrum, it is possible to obtain an optical wavelength multiplexing/de-multiplexing circuit in which an optical signal has a small deterioration even when passing therethrough many times or a loss variation is small for the wavelength fluctuation of the optical signal.
Further, for reducing (athermalizing) the temperature dependence of the transmission wavelength in the MZI-synchronized AWG, it is necessary to reduce the temperature dependence of the transmission wavelength in each of the MZI and the AWG. Non-patent document 2 discloses the athermal MZI-synchronized AWG by employing the method disclosed by Patent document 1 to each of the MZI and the AWG.
FIG. 42 is a plan view showing an exemplary configuration 4200 of the athermal MZI-synchronized AWG type optical wavelength multiplexing/de-multiplexing circuit. The athermal MZI-synchronized AWG type optical wavelength multiplexing/de-multiplexing circuit 4200 is configured with an AWG part 4200a and an MZI part 4200b. The AWG part is provided with a first slab waveguide 4201, an arrayed waveguide 4202, a second slab waveguide 4203, a second input/output waveguide 4204, and a groove 4205 which is formed in the first slab waveguide 4201 and which is filled with the temperature compensation material. The MZI part 4200b is provided with a first input/output waveguide 4206, an optical coupler 4207, a first arm waveguide 4208, a second arm waveguide 4209, a directional coupler 4210, and a groove 4211 which is formed in the first arm waveguide 4208 and which is filled with the temperature compensation material. Further, each of the grooves 4205 and 4211 is divided into plural grooves and this is because the plural grooves can reduce a radiation loss more than a single groove.
A light wave having plural wavelengths is input into the first input/output waveguide 4206 of the MZI 4200 and branches into the first and second arm waveguides 4208 and 4209 by the optical coupler 4207, in which an optical path length difference causes a phase difference according to the wavelength. This light wave has interference between two waveguides disposed closely in the directional coupler 4210 and power is distributed between the two waveguides according to the phase difference (i.e., wavelength). Therefore, the position of the light wave collected at a terminal, where the directional coupler 4210 is connected to the first slab waveguide 4201 of the AWG 4200, is caused to vary periodically between the two waveguides according to the phase difference (wavelength). Meanwhile, the light wave input into the AWG 4200a from the directional coupler 4210 is provided with a phase difference according to the wavelength by the optical path length difference between the neighboring waveguides in the arrayed waveguide 4202, and the position of the light wave collected at a terminal of the second slab waveguide 4203 changes according to the phase difference (i.e., wavelength), and then the light wave having a desired wavelength is de-multiplexed into each of the input/output waveguides 4204.
Here, when the position of the collected light wave is changed between the two waveguides in the directional coupler 4210, the input position of the light wave into the first slab waveguide 4201 changes and also the optical path length to each of the arrayed waveguides changes. Therefore, without the change of the optical path length difference between the two waveguides in the arrayed waveguide 4202, the optical path length difference in the whole optical wavelength multiplexing/de-multiplexing circuit changes and the position of the light wave collected at the terminal of the second slab waveguide 4203 changes. This means that the position of the light wave collected at the terminal of the second slab waveguide 4203 can be adjusted by the optical path length difference between the first and second arm waveguides 4208 and 4209 in the MZI 4200b. That is, when the position change of the light collected at the terminal of the directional coupler 4210 of the MZI 4200b and the position change of the light collected at the terminal of the second slab waveguide 4203 of the MZI 4200a are set to be synchronized in a certain wavelength region, the position of the light collected at the terminal of the second slab waveguide 4203 does not change and accordingly it is possible to obtain a flat transmission spectrum characteristic in this wavelength region.
In the athermal MZI-synchronized AWG of FIG. 42, the AWG part 4200a is athermalized by the same method as in the athermal AWG of FIG. 40. Further, in the MZI part 4200b, when a length difference in the first arm waveguide from the second arm waveguide 4209 is set to be ΔI, the sum of the lengths divided by the groove 4211 in the first arm waveguide is designed so as to become an amount ΔI′ proportional to ΔI. At this time, the transmission wavelength λcMZI of the MZI 4200b is expressed by the following formula.λcMZI={nrΔI′+n′ΔI′}/m  Formula 2Here, nr is an effective refractive index in the first and second arm waveguides 4208 and 4209, n′ is a refractive index of the temperature compensation material, m is a diffraction order of the MZI 4200b, and nrΔI′+n′ΔI′ indicates an optical path length difference in the first arm waveguide from the second arm waveguide 4209. Here, the effective refractive index temperature coefficient of the arm waveguide is a which is the same as that of the arrayed waveguide and the slab waveguide, and the athermalized MZI 4200b is designed as ΔI′/(ΔI−ΔI′)=−α/α′, that is, ΔI′=ΔI/(1−α′/α). Therefore, the temperature-induced change of the optical path length difference in the first arm waveguide 4208 from the second arm waveguide 4209 is cancelled by the temperature-induced change of an optical path length difference in the temperature compensation material filled in the groove and the temperature dependence of the transmission wavelength is compensated. For the temperature compensation material, it is preferable to use a material in which α′ and α have different sign and also |α′| is sufficiently larger than |α|, as in the example of FIG. 40, and the material satisfying such conditions includes, for example, a silicone resin of an optical resin.
The center transmission wavelength of the MZI-synchronized AWG has approximately an average value of the transmission wavelength of the MZI and the center transmission wavelength of the AWG. In the MZI-synchronized AWG of FIG. 42, since the transmission wavelength of the MZI 4200b and the center transmission wavelength of the AWG 4200a are athermalized, the center transmission wavelength of the MZI-synchronized AWG is athermalized. Further, a method disclosed by following Non-patent document 1 also can be applied to the method athermalizing the AWG part of the athermal MZI-synchronized AWG.